This invention relates to a long flexible waveguide having a substantially rectangular cross-section which is used as the transmission line of microwaves.
Waveguides for the transmission lines of microwaves have hitherto had various cross-section shapes such as circular, rectangular and as an ellipse. Further, long flexible waveguides which can be wound on a drum type winder have been often used to simplify the construction work and enhancing the reliability owing to the reduction of connection points. Those of the waveguides which are elliptical or approximately elliptical in section, are advantageous in that the workability is comparatively good and that electric characteristics are not spoiled even by a bending deformation arising when the waveguide is wound on the winder. Since, however, the rectangular waveguide is used on the feeder line side of a transmitter-receiver close to an input terminal and an antenna, the conversion ratio becomes large in case of matching the rectangular waveguide and the waveguide of the elliptic or approximately elliptic section. This leads to the disadvantage that the wide frequency band is damaged. Moreover, the elliptical or approximately elliptical waveguide exhibits a narrow fundamental mode band and has higher-order modes generated than the rectangular waveguide, so that the frequency band is restricted. Comparing an elliptic waveguide and a rectangular waveguide respectively illustrated in FIGS. 1(a) and 1(b), the frequencies at which higher-order modes are generated are made equal, the attenuation of rectangular waveguide is smaller in quantity as shown in FIG. 2. In the aspects of the electrical characteristics and the the rectangular waveguide is far more meritorious. The rectangular waveguide, however, involves problems such that corrugation in the rectangular state is very difficult and the electric characteristics are conspicuously damaged due to the bending deformation in the case of the windable long article.
One prior-art flexible rectangular waveguide has the corrugating pitch made small and the chamfering radius of each corner of the rectangle made as small as possible in order to match it with a rigid rectangular waveguide. In this case, a mandrel as an inner mold is required for forming a thin-walled pipe which is corrugated at the small pitch. Besides, even with the mandrel inserting system, a rectangular shape which is excellent in both workability and electric characteristics has not yet been discovered, and a desired precision in the case of a long article cannot be secured.
Where the H-bend is adopted in the bending job at the time of laying a prior-art long flexible waveguide, not only the flexural rigidity is high but also a problem to be explained below arises. FIGS. 8(a) and 8(b) are referred to in the explanation. In the prior-art, it is impossible to perform the H-bend with empty hands, and hence, an expedient of bending a waveguide 4' along a disc 3' as illustrated in FIG. 8(a) is used. Herein, a great thrust c is exerted on the side of the waveguide 4' by the disc 3' as a reaction component force of two forces in directions a and b are applied to the waveguide 4'. Although a force in the direction c also appears in case of the E-bend, it becomes very large in the case of the H-bend on account of the high flexural rigidity. Further, in actuality, the waveguide 4' is unbalanced and turns in a direction of arrow R as illustrated in FIG. 8(b). This renders it very difficult to smoothly execute the H-bend. The H-bend of the waveguide 4' along the disc 3' is therefore made smooth in such way that, as shown in FIGS. 9(a) and 9(b), the waveguide 4' is supported by a pair of upper and lower support means to 5' to check the turning. As previously stated, however, high flexural rigidity is involved in the H-bend. For this reason, a coating member 2' is pressed between a metal tube 1' and the disc 3' by the thrust in the direction of arrow c as applied from the disc 3' to the waveguide 4'. Since especially top areas of the crests of the corrugation have intense forces concentrically exerted thereon, the parts of the coating member 2' that correspond to the top areas undergo internal strains and become thin as illustrated in FIG. 10. Where, in this manner, the coating member 2' is thinned at the parts corresponding to the top areas of the crests of the corrugation, it deteriorates from the thin parts and gives rise to early cracks etc. under severe service conditions of wind and rain, intense heat, intense cold etc. When using the waveguide outdoors over a long time period. As a result, it loses its protective function for the metal tube 1' which is the principal constituent of the waveguide 4'.
Further, a prior-art long flexible waveguide which is corrugated in the form of a helix or bellows is not deformable by twisting. Another drawback is that, when it is twisted beyond a predetermined angle, the electrical characteristics are degraded at once. The reason for the degradation is believed to be that, since the structure which is difficult to be deformed is forcibly twisted, a part of the flexible waveguide yields to cause a buckling-like phenomenon, so a place indicated at A in FIG. 13 is locally distorted. (The distortion is exaggerated and shown in FIG. 13 in order to facilitate the understanding, but it is in actuality difficult to be examined with the naked eye. When the local distortion is to the extent that it can be seen with the naked eye, the flexible waveguide is quite inferior in the electric characteristics and cannot be used.)